Add support for memory block events to CliContext.

More adjustments to CliContext.

- CliContext now listens for value node container events so that
commands can request such a wait as well
- Implement an event wait mechanism for commands to make use of.
Adjust CliStackTrace and CliPrintVariable accordingly.

Various improvements to CliContext.

- CliContext now tracks the current stack trace and frame if applicable.

- CliContext now carries a value node manager. This allows it to track
the variables in the currently active frame.

Debugger CliContext: More event handling, current thread

* Introduce the notion of a current thread. That's the one certain
commands will target (by default).
* Add more event handling in CliContext. There's now a queue of pending
events which are printed in the input loop at convenient times to
inform the user about what happened (new/deleted/stopped threads,
etc.).

Debugger: CliContext: Introduce the notion of events

* The input loop can now wait on abstract events, which other threads
(or even the input loop thread itself) can signal.
* Use the new mechanism in QuitSession().
* Also (with the exception of the SIGINT part) implement
WaitForThreadOrUser().

Debugger CLI: Move more stuff to and extend CliContext

* Move the libedit interface there and provide nicer to use methods.
* Also start adding utility methods for the input loop. It is going to
manage all interactions of the input loop with outside events.
* Fix the "quit" command. The user is now prompted what to do with the
debugged team and the input loop thread avoids reentering the input
loop.

Debugger CLI: Start to imbue CliContext with life

Debugger: Some basic work to get the CLI going

There's an input loop thread which reads and parses command lines and
the infrastructure for registering and executing commands. Currently
only "help" and "quit" commands are implemented.

Debugger CliContext: More event handling, current thread

* Introduce the notion of a current thread. That's the one certain
commands will target (by default).
* Add more event handling in CliContext. There's now a queue of pending
events which are printed in the input loop at convenient times to
inform the user about what happened (new/deleted/stopped threads,
etc.).

Debugger: CliContext: Introduce the notion of events

* The input loop can now wait on abstract events, which other threads
(or even the input loop thread itself) can signal.
* Use the new mechanism in QuitSession().
* Also (with the exception of the SIGINT part) implement
WaitForThreadOrUser().

Debugger CLI: Move more stuff to and extend CliContext

* Move the libedit interface there and provide nicer to use methods.
* Also start adding utility methods for the input loop. It is going to
manage all interactions of the input loop with outside events.
* Fix the "quit" command. The user is now prompted what to do with the
debugged team and the input loop thread avoids reentering the input
loop.

Debugger CLI: Start to imbue CliContext with life

Debugger: Some basic work to get the CLI going

There's an input loop thread which reads and parses command lines and
the infrastructure for registering and executing commands. Currently
only "help" and "quit" commands are implemented.